Optical pulse source with increased peak power

ABSTRACT

In at least one embodiment time separated pulse pairs are generated, followed by amplification to increase the available peak and/or average power. The pulses are characterized by a time separation that exceeds the input pulse width and with distinct polarization states. The time and polarization discrimination allows easy extraction of the pulses after amplification. In some embodiments polarization maintaining (PM) fibers and/or amplifiers are utilized which provides a compact arrangement. At least one implementation provides for seeding of a solid state amplifier or large core fiber amplifier with time delayed, polarization split pulses, with capability for recombining the time separated pulses at an amplifier output. In various implementations suitable combinations of bulk optics and fibers may be utilized. In some implementations wavelength converted pulse trains are generated. A method and system of the present invention can be used in time domain applications utilizing multiple beam paths, for example spectroscopy.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 13/413,304 filed Mar. 6, 2012, which claims benefit of ProvisionalApplication No. 61/449,955, filed Mar. 7, 2011. The above-notedapplications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to methods and systems for generating laser pulseswith high average power or high peak power, and is particularlyapplicable to time-domain spectroscopy such as pump-probe or terahertzmeasurement, where multiple beams carrying pulse trains are to beutilized, or where average power and/or pulse energy can be providedwithout undesirable nonlinear effects.

BACKGROUND

Utilization of pulsed laser sources has increased in industrial andscientific applications. In particular, applications of ultrashort lasertechnology have increased over the last few years in metrology, imagingand material processing applications. Fiber-based ultrashort systems arenow well established for numerous applications, and are particularlywell suited for high repetition rate applications at low-medium pulseenergy. However, in either passive or gain fiber, the peak power of theamplified pulse is constrained because of the pulse distortion andsignal shifting out of the gain spectrum caused by nonlinear effects,for example Raman shifting. Chirped pulse amplification is often used togreatly extend the capability of fiber systems. Pulses are temporallystretched, thereby lowering the peak power, then amplified andrecompressed. Such constraints also apply to other optical media as thepulse energy scales up, for example Nd: based bulk optical amplifiers.

The following patents, published patent applications, and publicationsrelate, at least in part, to fiber lasers and amplifiers, ultrashortlaser material processing, optical measurement techniques, and/orvarious arrangements for generating groups of laser pulses: U.S. Pat.No. 6,339,602; U.S. Pat. No. 6,664,498; U.S. Pat. No. 6,954,575; U.S.Pat. No. 7,088,878; U.S. Pat. No. 7,580,432; U.S. Patent ApplicationPub. No. 2002/0167581; U.S. Patent Application Pub. No. 2003/0151053;U.S. Patent Application Pub. No. 2005/0218122; U.S Patent ApplicationPub. No. 2010/0272137; WIPO Pub. No. 2009146671; Strickland and G.Mourou, Opt. Commun. 56, 219 (1985). H. Hofer et. al., Opt. Lett. 23,1840 (1998); M. E. Fermann et al., Phys. Rev. Lett., 84, 2000 (2010).

Various applications require multiple beams or pulse trains. In suchapplications, pulses in the multiple beams may have a well-definedrelative time interval, requiring some level of synchronization. Timedomain measurements are an example. More specifically, with optical timegating or correlation techniques, a first beam is used for opticalinteraction with a sample, and a second beam is used for a time gatingor correlation function. Specifically, for an ultrashort measurement,synchronization is needed to obtain the desired time resolution.Terahertz spectroscopy, optical pump-probe spectroscopy and other timegated imaging processes utilizing an ultrashort pulse laser fall intothis application category.

Conventional laser-based systems used for such applications are oftendesigned to create pulses with sufficiently high energy, and tosubsequently divide the beam into multiple beam paths in the applicationsystem.

Amplification of high intensity optical pulses in an optical fiber andother gain media, for example regenerative amplifiers, ultimatelyrequires consideration of nonlinearity. Often the pulse energyconstraint results in limited average power. Increasing the averagepower without loss of pulse energy would be a useful improvement forhigh peak power pulse laser systems.

Therefore, a need exists to extend the peak power capability of pulsedlaser sources, including fiber based systems, regenerative amplifiers,thin disk lasers, and the like.

SUMMARY OF THE INVENTION

In one aspect the present invention features a method to reducenonlinear pulse distortion and to increase the available average powerof a pulsed laser source.

Amplifying pulses distributed in the time domain reduces nonlineareffects which would otherwise be induced by high peak power. If the timedistributed pulses have different polarization states, the pulses can beeasily separated.

In various embodiments a pulse is split into distinct polarizationstates prior to or during amplification. The resulting split pulseportions may have orthogonally linear polarizations. The distinctpolarization states provide for easy extraction of the synchronizedpulse trains into multiple beams utilizing relatively simplepolarization sensitive devices. A time separation between pulse pairscan be introduced during propagation in an optical medium, with timeseparation being greater than a pulse width.

In some embodiments a second splitting unit may be used to recombine twopolarization split, time separated pulses by propagating a beam in anopposite direction to compensate the time delay. In some implementationsthe second splitting unit may include the same optical components as thefirst unit.

In various embodiments, polarization split, time separated pulses aregenerated in an active medium before the pulses are amplified to athreshold at which unwanted nonlinear effects occur.

In at least one embodiment pulses are temporally split prior toamplifying with one or more amplifier stages.

In at least one embodiment both the polarization splitting and delaygeneration are implemented in an all-fiber arrangement, which mayinclude active or passive polarization maintaining (PM) fiber. Forexample, with PM optical fiber, a polarization splitter and delaygenerator may be integral with an active/passive PM fiber medium and notrequire separate components.

In a fiber laser configuration the oscillator output can be separatedinto two or more beams, with an optional delay stage for each beam path,before being combined and injected into the amplifier fiber. Ifsufficient delay is provided beyond the pulse width of the pulse thenthe pulses are amplified without interference, thereby preserving theduration of each pulse.

In various embodiments utilizing linearly polarized pulses, the splitbeams may be manipulated so that the polarization is linear but withorthogonal polarization states. After amplification in a gain medium, apolarization sensitive device can easily separate the pulses forsubsequent operations. In various embodiments utilizing fiber laserswith polarization maintaining fibers, the splitting in time andpolarization can be further simplified by utilizing the group velocitydifference of the two orthogonally linear polarized pulses in the fiber.In this example, the input polarization to the PM fiber is set so thatslow and fast axis polarizations are simultaneously excited. After asufficient propagation length in the fiber the pulses in the twopolarization components will be separated, preferably by more than thepulse width of the input pulse. At least two amplified pulse trains canthen be extracted by a polarization component.

In at least one embodiment, the two laser pulses can be coherently orincoherently combined to generate a single laser pulse with higher peakpower output than a non-linear threshold of at least one medium.

In at least one embodiment, laser pulse trains in both polarizationstates, or in one polarization state, can be input to an opticalnonlinear device to convert a first wavelength to a second wavelength.

In one application the laser output comprises two physical beams, andthe corresponding pulse trains are synchronized. Non-limiting examplesof applications include polarization based material modification andprocessing, time domain spectroscopy and imaging based on the timedomain information, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically illustrates conventional pulse propagation in amedium which results in pulse distortion. FIG. 1B schematicallyillustrates time separated pulses with different polarization states inthe medium and pulses being combined into a single output beam withhigher power or pulse energy, and negligible distortion.

FIG. 2 schematically illustrates an example of a pulsed laser sourceutilizing PM fibers.

FIG. 3A schematically illustrates an example of a polarization splittingunit or combining unit.

FIG. 3B schematically illustrates an example of a delay generator withcontrollable delay.

FIG. 4A schematically illustrates another example of a polarizationsplitter or reciprocal combiner.

FIG. 4B schematically illustrates two examples of delay generators.

FIG. 5 is a plot showing a measured autocorrelation function (ACF)corresponding to two time separated and amplified pulses.

FIG. 6 illustrates an exemplary system for an application where twobeams and two separate pulses are used to obtain time domaininformation.

FIG. 7 schematically illustrates time separated, polarization splitpulses in a plurality of optical paths and wavelength conversion of oneor more pulses. A pre-determined time separation provides forsynchronization.

DETAILED DESCRIPTION

In at least one embodiment the available average output power of anamplifier is increased without substantial increase of pulse energy.

In at least one embodiment one pulse is split into a pair of pulses,each having a different polarization state. A relative delay between thepolarization split pulses is generated and temporally separates thepulses during propagation in a medium, for example a passive opticalmaterial or an active, amplifying gain medium. In some embodiments thepulses may separate, at least in part, during amplification in a gainmedium.

Some pulsed laser sources, such as fiber laser amplifiers utilizing PMfibers, preserve the polarization states. When excitation is sufficient,each polarization state can independently propagate in the laser source.

By way of example, FIGS. 1a and 1b compare pulse propagation 110 in amedium of a conventional laser system with propagation 120 of timeseparated pulses in accordance with one implementation of the presentinvention. FIG. 1a shows a laser pulse 105 a propagating in a portion ofmedium 101, which can be a gain medium or passive medium. This exampleillustrates output pulse distortion 105-b induced by nonlinear effectswithin the medium. Such effects may be manifested by pulse breakup,noise, and other distortion. As discussed above, distortion levelscaused by non-linear effect(s) constrain the achievable peak power forvarious laser-based applications. It is known that Raman shifting and/orself-phase modulation can significantly transform a Gaussian-like inputpulse to a substantially distorted temporal pulse shape similar to105-b, for example. Non-linear effects have been exploited to improvepulse quality, for example as disclosed in U.S. Pat. No. 7,414,780,where output pulse quality was improved with an increase in self-phasemodulation. Nevertheless, further increase in the available peak poweris beneficial in such a system, at a power level when substantialdegradation of output pulse quality is observed.

FIG. 1b schematically illustrates how laser pulses with orthogonalpolarizations 115-a can be used to double the available output maximumpulse energy in a pulsed laser system. As schematically illustrated inFIGS. 1a and 1b , an output pulse 115-b having increased peak power mayhave a pulse temporal shape similar to the temporal shape of a pulse105-a generated with the seed source, e.g.: prior to a time and locationat which the input (seed) pulse becomes distorted. If two laser pulseswith orthogonal polarization are temporally separated prior to or duringpropagation in the medium 101, each pulse can separately reach themaximum pulse energy supported by the medium before onset of non-lineareffects. With pre-determined beam profile and pulse shape, the maximumpulse energy can be determined. An optional combiner after the medium101 compensates the temporal delay between these two pulses, andcombines the two pulses into a single pulse. A resultant pulse 115-b hastwice the maximum energy of a single pulse (before separation into thepulse pair).

FIG. 2 illustrates one example of the polarization splitting andcombining operation in an exemplary fiber-based laser system. In thisexample, the laser source includes a seed laser, which may include amode locked laser oscillator coupled to PM fiber. In this configurationthe seed has a pure linear polarized output, depicted with the verticalarrow. The seed laser is configured to provide laser pulse trains withultrashort pulses. The pulse trains are amplified with a laser amplifier220 having a doped gain fiber as a gain medium, also based on PM fiber

One efficient way to split each seed pulse into two orthogonalpolarization states is to arrange PM fiber by splicing two sections ofPM fiber 210, 215 between the seed source and the amplifier 220 with anangular shift. Schematic cross sectional views 210-a, 215-a illustratethe relative angular displacement of the PM fiber polarization axes. Theaxes are determined, at least in part, by the birefringent materialdisposed in the fiber cladding which partially surrounds the fiber core.Such splicing may be carried out automatically with commerciallyavailable splicing machines and software.

Assume polarization of the seed laser is parallel to the slow axis ofthe input fiber. Then, with an angular shift of θ, the power in the fastaxis and the slow axis of the output fiber are I₀ cos²θ, and I₀ sin² θ,respectively. In this particular example, the temporal delay betweenthese two polarization states is applied by the PM fiber in an amplifierportion of the pulsed laser system. The PM fiber has birefringence δn onthe order of 10⁻⁴ between its fast and slow axes. This results atemporal delay from a few hundreds of femsosecond (fs) to about apicosecond (ps) in each meter of PM fiber. If the amplifier portion hasa PM fiber length of a few meters, several ps of temporal delay results.This temporal delay is usually sufficient to separate two ultrashortpulses.

Notably, in this example, the temporal delay between the pulses of apulse pair varies in the laser amplifier 220 and two pulses completelyseparate near the end portion of the amplifier. This variable delay isnot detrimental because laser power increases during the propagationalong with the separation. Thus, although the pulse is not temporallyseparated immediately upon injection to the amplifier, the total powerof each pulse is still below the maximum peak power because both pulseshave reduced pulse energy, and the pulses separate before the sum of thepulse powers exceeds a threshold for non-linear effects. Maximum pulseseparation is to be obtained at or near the output end of the amplifier,where the polarization split, time delayed amplified pulses 230 havemaximum energy.

It is to be understood that FIG. 2 illustrates one of many possibleimplementations. For example, the seed laser may not be linearlypolarized, it can be in other polarization states, such as circularlypolarized, elliptically polarized, or even non-polarized, depolarized orpartially polarized. In various embodiments a seed pulse in a purepolarization state is more desirable for controlling the ratio betweentwo orthogonal polarization states in the amplifier, and well suited forcoherent combining after amplification. The required temporal separationneed not occur in a laser amplifier. The separation can also be appliedin a passive medium, laser oscillator, and/or amplifier,

In various implementations the polarization splitting can be done usingfree space coupling rather than fiber splicing. A polarization splittermay include a mechanism for relative rotation of a fiber or waveplate tofurther control and align the polarization. A delay can also be setusing bulk optical components as will be discussed below with respect toFIG. 3 and FIG. 4. Furthermore, a delay can be variable rather thanfixed.

Moreover, the amplifier need not be a fiber amplifier. The medium can beany suitable medium, passive or active, which limits the maximum pulseenergy, and may be disposed outside of the laser source itself.

FIG. 3a illustrates an exemplary polarization splitting unit (orreciprocal combining unit with a reversed beam). A slab of birefringentcrystal 310, which has different refractive indices (n_(s), n_(p)) forthe two orthogonal polarization states, can be used to split a pulse orcombine polarization split, time separated pulse pairs. LiNbO₃, forexample, has δn=0.085 between the ordinary and extraordinarypolarization states. A 3.5 mm thick portion of LiNbO₃ can provide about1 ps temporal delay between the pulse pair. As illustrated in FIG. 3b ,crystals 330-a, 330-b, each having a prism shape, can change the opticalpath length, and thus control the temporal delay by translating one orboth crystals.

FIG. 4a illustrates yet another example of a polarization splitting unit400 (or reciprocal combining unit). A polarized beam splitter PBS isused to separate the laser pulse into different polarization states anddirect them along different paths (arms). Delay generating units 410-a,410-b are configured to control the temporal relationship between thepair of pulses.

FIG. 4b illustrates two examples of devices suitable for delaygeneration. One is a linear delay line 420 and the other one comprisestwo prisms 430-a, 430-b, similar to the components illustrated in FIG.3b , which can be used to control the optical path length. Such devicesmay be used alone or in combination. Also, although a two-fold increasein peak power is available with two pulses, additional paths may be usedto form more than two time separated pulses in a desired time sequence,thereby providing for further increased output peak power.

The pulses may then be recombined in a combiner, which may comprise anysuitable combination of bulk and fiber optics, for example asschematically illustrated in FIGS. 3-4. As also discussed above, areversed path may be used to combine the pulses in FIG. 4 a.

By way of example, the output pulses may be used with suitable beamconditioning optics as an input to one or more of a downstream bulksolid state gain medium, a large mode multimode amplifier fiber (MMFA)capable of providing substantially fundamental mode output, large coreleakage channel amplifier fiber (LCF design), photonic crystal amplifierfiber (PCF design), a high power coherent amplifier array, and/or otherhigh peak power gain media. A combiner comprising bulk optics may beimplemented at an output of the downstream gain medium to form a singlepulse with increased peak power (not separately shown). Similarly, invarious embodiments, the MMFA, LCF, or PCF may comprise PM fiber andprovide at least a portion of the splitting and delay generation.

Moreover, various combinations of the above components andconfigurations may be utilized for any application where polarizationsplit, time delayed pulse pairs can be advantageous to improve the peakand/average output power capability of an optical medium.

Different beams and pulse trains can be used in a time domainmeasurement technique such as terahertz spectroscopy or pump and probespectroscopy. A schematic illustration of a time domain measurementsystem 600 is shown in FIG. 6. A laser source 605 as discussed abovedelivers two beams and pulse trains, where one pulse train can befurther delayed in time relative to the other pulse train via delay line610. Sample 1 620 interacts with a first pulse train while the otherpulse train interacts with the first pulse train in Sample 2 630.

By way of example, Sample 1 can be a terahertz emitter and Sample 2 thetime gating element of the terahertz wave interacting with the gatingpulse which is time delayed. Pump and probe or similar opticalcorrelation techniques share a similar principle of operation, in whichthe time gating of the optical signal is induced in a sample.

In some embodiments one or both pulse trains may be wavelengthconverted. By way of example, a harmonic converter, a Raman shifter, oroptical parametric amplifier (OPA) may be utilized for wavelengthconversion. The resultant output may be combined and time synchronized,or processed separately. The wavelength converter may be disposed eitherbefore or after a beam combiner.

FIG. 7 schematically illustrates a further application of twopolarization split, time separated pulses. In this example, an inputprovides polarization split, time separated pulses as discussed above.The pulses are directed to separate optical paths. In a first opticalpath a pulse is frequency doubled with second harmonic generator SHG.The second pulse, propagating in a second optical path, may have apre-determined delay and therefore can be time-synchronized with thefrequency converted pulse. Many variations are possible, for examplewavelength converting in the second optical path, combining, and thelike.

In one experiment a Raman soliton laser amplifier was used in aconfiguration similar to that illustrated in FIG. 2. A mode locked fiberlaser oscillator generated a central wavelength about 1560 nm, and wasconfigured with a PM fiber pigtail. The output laser polarization wasaligned with the slow axis. The seed pulse was injected into a laseramplifier by splicing the output PM pigtail to a PM gain fiber. 45degree shifting was applied during splicing. Thus, equal power was splitinto the fast and slow axis in the laser amplifier. The gain fiber waspumped with a laser diode. In this example, a Raman soliton was formedduring amplification. The output Raman soliton had pulse duration ofabout 100 fs. With only one polarization seed, the Raman soliton pulseenergy saturated to maximum pulse energy. With two polarization seedinputs, each polarization produced a Raman soliton and saturated to themaximum pulse energy. As a result, the total Raman soliton power wasdoubled.

FIG. 5 shows the measured autocorrelation function (ACF) of theamplified laser output. The ACF clearly shows a double pulse structurewith 1.7 ps separation, which completely separates the pulse pair. It ispossible if the Raman soliton were not used in the experiment, the timeseparation might have been slightly different due to the difference ingroup velocity dispersion of each polarization axis associated with theRaman generation process. However, such variation is not significant indemonstrating time separation of the pulses during the propagation in aPM fiber.

Thus, the invention has been described in several embodiments. It is tobe understood that the embodiments are not mutually exclusive, andelements described in connection with one embodiment may be combinedwith, or eliminated from, other embodiments in suitable ways toaccomplish desired design objectives.

At least one embodiment includes a pulsed laser system. The systemincludes a seed source for generating a pulse. A polarization splittersplits a pulse from the seed source into different polarization states,thereby forming polarization split pulses. A delay generator receivesthe polarization split pulses and generates time separated pulses, eachpulse having a different polarization state. The system includes amedium in which the time separated pulses having the differentpolarization states propagate, wherein a peak power and energy of eachof the time separated pulses are individually sufficiently low to avoidsubstantial distortion of a pulse output from the medium. The power ofthe time separated pulses, if combined in the medium, would exceed anon-linear threshold of the medium. A combiner receives the timeseparated pulses from the medium and substantially re-combines the timeseparated pulses to form an output pulse having increased peak power.

In any or all embodiments a medium may include a polarization splitter.

In any or all embodiments a medium may include a fiber gain medium.

In any or all embodiments a medium may include a polarizationmaintaining (PM) amplifier fiber.

In any or all embodiments a seed source may generate linearly polarizedpulses.

In any or all embodiments a seed source may include a mode locked fiberoscillator.

In any or all embodiments at least a portion of a polarization splittermay include a polarization sensitive, bulk optic.

In any or all embodiments a delay generator and a polarization splittermay be coupled with optical fiber.

In any or all embodiments at least a portion of a delay generator mayinclude an active or passive PM fiber.

In any or all embodiments a seed source may include a mode-locked fiberoscillator having at least one polarization maintaining (PM) fiber.

In any or all embodiments a polarization splitter or combiner mayinclude at least one polarized beam splitter, which splits laser pulseswith each polarization state into separate arms, and a delay generatordisposed in at least one arm.

In any or all embodiments a pulse width generated by the seed source maybe shorter than the time spacing between adjacent, time separatedpulses.

In any or all embodiments a medium may include a plurality of PM fibers,including at least one active PM fiber.

In any or all embodiments an active fiber may include a multimodeamplifier fiber capable of providing a substantially fundamental modeoutput, a leakage channel amplifier fiber, a photonic crystal amplifierfiber, or a combination thereof.

In any or all embodiments an active fiber may be capable of Ramansoliton generation with multiple polarization states.

In any or all embodiments a wavelength of a seed source or Raman solitonwavelength may be in an anomalous dispersion regime.

In any or all embodiments a medium may include a bulk, solid state or aregenerative amplifier gain medium.

In any or all embodiments a polarization splitter may include PM fiberconfigured such that an input beam to the polarization splitter iscoupled into both the fast and slow axes of the PM fiber.

In any or all embodiments an input beam may be coupled to the PM fibervia fiber splicing, and the polarization splitting may be controlled byangular offset in the splicing.

In any or all embodiments a polarization splitter may be controllablewith relative rotation of a fiber or waveplate.

In any or all embodiments a seed pulse or output pulse may have a pulsewidth in the fs-ps regime.

In any or all embodiments a medium may include an amplifier fiber, and aseed beam may be coupled into the amplifier fiber with at least one bulkoptical element and free space coupling.

In any or all embodiments a medium may include an amplifier fiber, and aseed beam may be coupled into the amplifier fiber with fusion splicing,and polarization splitting may be controlled by angle offset insplicing.

In any or all embodiments the medium may include a gain medium, and adelay between pulses in different polarization states may be comparableor larger than a pulse duration in the gain medium.

In any or all embodiments a medium may include an amplifier fiber, and adelay between laser pulses may be longer than the laser pulse width inat least in one portion of the laser amplifier.

In any or all embodiments a PM fiber may be configured as a delaygenerator, the PM fiber comprising one or both of active and passivefiber.

In any or all embodiments a medium may be capable of amplifying laserpulse trains with orthogonal polarization states, and capable ofgenerating a Raman shift with the polarization states.

In any or all embodiments a Raman soliton may optionally be generatedwith orthogonal polarization states, in a laser amplifier.

In any or all embodiments a seed laser may produce a pulse width in therange from about 100 fs to a few ps, with a corresponding spectralbandwidth of at least a few nm.

In any or all embodiments at least one portion of a medium may includelarge mode area fiber comprising Yb/Er co-doped double cladding fiberwith a core diameter of at least about 15 μm.

In any or all embodiments a polarization splitter and a delay generatormay introduce delay larger than a pulse duration in the medium.

In any or all embodiments at least a portion of a combiner may beconfigured with identical components of a polarization splitter and adelay generator for reciprocal operation.

In any or all embodiments an output pulse having increased peak powermay have a pulse temporal shape similar to the temporal shape of a pulsegenerated with the seed source.

In any or all embodiments a peak power and energy of each of the timeseparated pulses may be sufficiently low to avoid substantial distortionof a pulse during propagation in the medium and when output from themedium.

In any or all embodiments a pair of time separated pulses may begenerated with a delay generator.

At least one embodiment includes a pulsed laser system. The systemincludes a seed source for generating a pulse. A polarization splittersplits a pulse from the seed source into different polarization states,thereby forming polarization split pulses. A delay generator receivesthe polarization split pulses and generates time separated pulses, eachpulse having a different polarization state. The system includes amedium in which the time separated pulses having the differentpolarization states propagates, wherein a peak power and energy of eachof the time separated pulses is sufficiently low to avoid substantialdistortion of a pulse output from the medium. The power of the timeseparated pulses, if combined in the medium, would exceed a non-linearthreshold of the medium. A bulk, solid state amplifier disposeddownstream from the medium receives the time separated pulses therefrom,and generates amplified time separated pulses. The system includes acombiner that receives the amplified time separated pulses from thebulk, solid state amplifier and substantially re-combines the amplifiedtime separated pulses to form an output pulse having increased peakpower.

In any or all embodiments both of a polarization splitter and a delaygenerator may be configured with PM maintaining fibers.

In any or all embodiments a medium may include PM optical fiber, and apolarization splitter and a delay generator may be integral with themedium.

In any or all embodiments a medium may include at least one amplifierfiber.

In any or all embodiments at least one amplifier fiber may includesingle mode, polarization preserving fiber.

In any or all embodiments at least one amplifier fiber may include amultimode amplifier fiber capable of providing a substantiallyfundamental mode output, leakage channel amplifier fiber, a photoniccrystal amplifier fiber, or a combination thereof.

In any or all embodiments at least one amplifier fiber may be capable ofRaman soliton generation.

In any or all embodiments a polarization splitter, a delay generator,and a medium may include optical fiber and no bulk optical components.

In any or all embodiments a pair of time separated pulses may begenerated with a delay generator.

At least one embodiment includes a pulsed laser system. The systemincludes a seed source for generating a pulse. A polarization splittersplits a pulse from the seed source into different polarization states,thereby forming polarization split pulses. A delay generator receivesthe polarization split pulses and generates time separated pulses, eachpulse having a different polarization state. The system includes anoptical amplifier in which the time separated pulses having thedifferent polarization states propagates, wherein a peak power andenergy of each of the time separated pulses is sufficiently low to avoidsubstantial distortion of a pulse output from the optical amplifier. Thepower of the time separated pulses, if combined in the opticalamplifier, would exceed a non-linear threshold of a gain medium of theoptical amplifier. The optical amplifier generates amplified timeseparated pulses as an amplifier output. The system includes a combinerthat receives the amplified time separated pulses from the opticalamplifier and substantially re-combines the amplified time separatedpulses to form an output pulse having increased peak power.

In any or all embodiments an optical amplifier may include at least onefiber amplifier.

In any or all embodiments an optical amplifier may include a large coreamplifier configured as one or more of a multimode fiber amplifier(MMFA) capable of providing a substantially fundamental mode output, aleakage channel fiber amplifier (LCF), a photonic crystal fiberamplifier (PCF), or a combination thereof.

In any or all embodiments a bulk solid state amplifier may be disposedbetween the optical amplifier and the combiner.

In any or all embodiments an output pulse having increased peak powermay have a pulse temporal shape similar to the temporal shape of a pulsegenerated with the seed source.

In any or all embodiments a peak power and energy of each time separatedpulse may be sufficiently low to avoid substantial distortion of a pulseduring propagation in the medium and when output from the medium.

In any or all embodiments a pair of time separated pulses may begenerated with a delay generator.

At least one embodiment includes a pulsed laser system. The systemincludes an input providing time separated pulses, each pulse having adifferent polarization state. The system includes a medium in which thetime separated pulses propagates, wherein a peak power and energy ofeach of the time separated pulses is sufficiently low to avoidsubstantial distortion of a pulse output from the medium. The system isfurther configured such that the time separated pulses propagate inseparate optical paths, thereby providing for synchronization of timeseparated pulses.

In any or all embodiments a wavelength converter may be disposed in afirst optical path and receives a first pulse from the medium, andconverts the wavelength of the first pulse from the medium to a firstconverted wavelength.

In any or all embodiments a second wavelength converter may be disposedin a second optical path and receives a second pulse from the medium,and converts the wavelength of the second pulse from the medium to asecond converted wavelength.

In any or all embodiments an input may include a gain medium comprisingsubstantially all-fiber and no bulk optical components in the gainmedium.

In any or all embodiments an input may include a delay generator, and atleast a portion of a delay generator may include bulk optics.

In any or all embodiments bulk optics may include a pair of prism-shapedbirefringent crystals, wherein the thickness and/or relative position ofone or both prism-shaped crystals provides for adjustable delay.

At least one embodiment includes a pulsed laser system. The systemincludes a means for generating time separated pulses, each pulse havinga different polarization state. The system includes a medium in whichthe plurality of time separated pulses propagates, wherein a peak powerand energy of each of the time separated pulses is sufficiently low toavoid substantial distortion of a pulse output from the medium, whereinthe system is configured such that the time separated pulses propagatein separate optical paths, thereby providing for synchronization of timeseparated pulses.

In any or all embodiments the means for generating and at least aportion of the medium may include optical fiber and no bulk opticalcomponents.

In any or all embodiments the power of time separated pulses, ifcombined in the medium, may exceed a non-linear threshold of the medium.

In any or all embodiments an output pulse having increased peak powermay have a pulse temporal shape similar to the temporal shape of a pulsegenerated with the seed source.

In any or all embodiments a peak power and energy of each time separatedpulse may be sufficiently low to avoid substantial distortion of pulsesduring propagation in the medium and when output from the medium.

Thus, while only certain embodiments have been specifically describedherein, it will be apparent that numerous modifications may be madethereto without departing from the spirit and scope of the invention.Further, acronyms are used merely to enhance the readability of thespecification and claims. It should be noted that these acronyms are notintended to lessen the generality of the terms used and they should notbe construed to restrict the scope of the claims to the embodimentsdescribed therein.

What is claimed is:
 1. A pulsed laser system, comprising: an inputproviding time separated pulses having a temporal delay therebetween,each pulse having a different polarization state; and a single medium inwhich said time separated pulses propagates, wherein a peak power andenergy of each pulse is sufficiently low to avoid substantial distortionof a pulse output from said medium, wherein the power of the timeseparated pulses, if combined in said medium without said temporaldelay, would exceed a non-linear threshold of said medium, wherein saidsystem is configured such that the time separated pulses output fromsaid medium propagate in separate optical paths providing forsynchronization of time separated pulses.
 2. The pulsed laser system ofclaim 1, comprising a wavelength converter disposed in a first opticalpath and receiving a first pulse from said medium, and converting thewavelength of said first pulse from said medium to a first convertedwavelength.
 3. The pulsed laser system claim 2, wherein said systemcomprises a second wavelength converter in a second optical path andreceiving a second pulse from said medium, and converting the wavelengthof said second pulse from said medium to a second converted wavelength.4. The pulsed laser system of claim 1, wherein said input comprises again medium comprising substantially all-fiber and no bulk opticalcomponents in said gain medium.
 5. The pulsed laser system of claim 4,wherein said input comprises a delay generator, and at least a portionof said delay generator comprises bulk optics.
 6. A pulsed laser system,comprising: a seed source generating a pulse; a polarization splitter tosplit said pulse into different polarization states, thereby formingpolarization split pulses; a delay generator which receives saidpolarization split pulses and generates time separated pulses, saiddelay generator configured to control a temporal relationship betweensaid polarization split pulses and to provide a temporal delaytherebetween, each pulse having a different polarization state; a singlemedium in which said time separated pulses having said differentpolarization states propagate, wherein a peak power and energy of eachof said time separated pulses is sufficiently low to avoid substantialdistortion of a pulse output from said medium, wherein the power of thetime separated pulses, if combined in said medium without said temporaldelay, would exceed a non-linear threshold of said medium; and acombiner that receives said time separated pulses from said medium andsubstantially re-combines the time separated pulses and compensates saidtemporal delay to form an output pulse having increased peak power,wherein said polarization splitter and said delay generator introducedelay larger than duration of said pulse generated by said seed source.7. The pulse laser system of claim 6, wherein said polarization splittercomprises PM fiber operably arranged such that an input beam to said PMfiber is coupled into both the fast and slow axes of said PM fiber, viafiber splicing, and said polarization splitting is controlled by angularoffset in splicing.
 8. A pulse laser system, comprising, a seed sourcegenerating an optical pulse; an input providing polarization splitoptical pulses, each polarization split pulse having a differentpolarization state; and one, or a series, of optical fibers at least oneof which comprises a gain fiber having a gain medium, said timeseparated pulses propagating in said one or series of optical fibers andexhibiting variable and increasing temporal separation in at least oneportion therein, said time separated pulses having said differentpolarization states, wherein a peak power and energy of each of saidtime separated pulses is sufficiently low to avoid substantialdistortion of a pulse output from said one or series of optical fibers,wherein the power of the time separated pulses, if combined in said gainmedium without said temporal separation, would produce substantialdegradation of output pulse quality; and a combiner that receives saidtime separated pulses from said one or series of optical fibers andsubstantially re-combines the time separated pulses and compensates saidtemporal delay to form an output pulse having increased peak power. 9.The pulse laser system of claim 8, wherein said input is integral withsaid one or series of optical fibers and said polarization split pulsesare generated therein.
 10. The pulse laser system of claim 8, whereinpulses provided by seed source are linearly polarized.
 11. The pulselaser system of claim 8, wherein pulses provided by seed source arecircularly polarized, elliptically polarized, or non-polarized,depolarized or partially polarized.
 12. The pulse laser system of claim8, wherein said input comprises a PM fiber pigtail optically connectedto said seed source and arranged to receive said pulse, and operablyarranged such that power is distributed in each of the fast and slowaxes of said one or more fibers disposed downstream from said input. 13.The pulse laser system of claim 8, wherein said input is disposeddownstream from said seed source.
 14. The pulse laser system of claim 8,wherein said input comprises a polarization splitter and a delaygenerator.
 15. The pulse laser system of claim 8, wherein said inputcomprises a linearly polarized beam, and wherein at least one fiber is aPM fiber, said input and said one or series of optical fibers operablyarranged such power is distributed in each of the fast and slow axes ofsaid PM fiber.
 16. The pulse laser system of claim 8, wherein said gainfiber is a PM amplifier fiber.
 17. The pulse laser system of claim 8,wherein at least one fiber supports multiple modes.
 18. The pulse lasersystem of claim 8, wherein at least one fiber is single mode.
 19. Thepulse laser system of claim 8, wherein said one or series of fiberscomprises: a large mode multimode amplifier fiber (MMFA) capable ofproviding substantially fundamental mode output, large core leakagechannel amplifier fiber (LCF design), photonic crystal amplifier fiber(PCF design),


20. The pulse laser system of claim 8, wherein if the power of said timeseparated pulses were combined in said medium without said temporaldelay, a non-linear threshold of said medium would be exceeded, and oneor more nonlinear effects would substantially degrade output pulsequality.
 21. The pulse laser system of claim 8, wherein if the power ofsaid time separated pulses were combined in said medium without saidtemporal delay, output pulse quality would be substantially degraded bypulse breakup or noise.
 22. The pulse laser system of claim 8, whereinone or both of polarization splitting or delay generation areimplemented in an all-fiber arrangement, including splices betweenfibers.
 23. The pulse laser system of claim 8, wherein said series offibers are joined by fiber splices.
 24. A pulse laser system,comprising, a seed source generating a linearly polarized optical pulse;one or more optical fibers, including at least one polarizationmaintaining (PM) optical fiber optically connected to said seed sourceand operably arranged to generate polarization split optical pulseshaving different polarization states and a temporal delay therebetween;and a combiner that receives said time separated pulses as output fromsaid one or more optical fibers, and substantially re-combines the timeseparated pulses and compensates said temporal delay to form an outputpulse having increased peak power, wherein a peak power and energy ofeach of said time separated pulses is sufficiently low to avoidsubstantial distortion of a pulse output from said one or more opticalfibers, wherein the power of the time separated pulses, if combined insaid one or more fibers without said temporal delay, would substantiallydegrade output pulse quality.
 25. The pulse laser system of claim 24,wherein said seed source comprises a passively mode locked laser. 26.The pulse laser system of claim 25, wherein said passively mode lockedlaser includes a fiber gain medium.
 27. The pulse laser system of claim24, wherein passively mode locked fiber laser comprises PM fiber. 28.The pulse laser system of claim 24, wherein said at least one PM fibercomprises a PM fiber amplifier.
 29. The pulse laser system of claim 24,wherein at least one of said one or more optical fibers comprises a gainmedium.